Bob Battalio, PE Chief Engineer, ESA September 8, 2016

Transcription

1 RELATING FUTURE COASTAL CONDITIONS TO EXISTING FEMA FLOOD HAZARD MAPS Technical Methods Manual Bob Battalio, PE Chief Engineer, ESA September 8, 2016 FMA 2016 Sacramento, California DWR-OST-SIO PILOTING NON-STATIONARY APPROACHES TO FLOODPLAIN MANAGEMENT: SUPPORTING LOCAL COMMUNITIES AND INFORMING NATIONAL POLICY

2 1. Overview 2. Report Organization 3. Methods with Examples 4. Recommendations Presentation Outline

3 1. Overview - Background NOAA grant to DWR and OST and SIO Needs Assessment (OST) Quick Guide Update (Ford) Future coastal total water levels (SIO) Comprehensive Report (OST/DWR/SIO) DWR-OST: Technical Methods Manual (ESA + SIO+TMMC) Focus Group Technical Methods Manual Committee (TMMC)

4 1. Overview TMM Development and Attributes Several Drafts, comments, revisions Dec 2015 extensive comments, reorganization May 2016 editorial comments July 2016 final draft August 2016 final report Collaborative with detailed reviews from Technical Methods Manual Committee as well as the DWR-OST-SIO grant team Focused on local government informed coastal planning Distills recent climate change science (SIO future projections) and integrates with contemporary engineering practice (FEMA mapping and coastal hazards quantification) Quantifies historic and forecast extreme water levels and waves for California (3 regions South, Central and North) Methods accommodate information not yet available, with recommendations

5 2. TMM Organization - Contents TABLE OF CONTENTS Technical Methods Manual 1. Introduction 2. Coastal Flooding Parameters 3. Methods to Adjust FEMA Maps for Sea Level Rise 4. Examples 5. Recommendations 6. References 7. Acknowledgements Appendices A. An Overview of FEMA Flood Insurance Studies and Flood Insurance Rate Maps B. Scripps Institution of Oceanography (SIO) Future Waves and Water Levels B1. Extreme Value Analysis on Scripps Institute of Oceanography Data B2. Extreme Value Analysis on Observed Data C. Other Sea Level Rise Hazard Mapping Studies and Sources

6 3. Methods and Examples - Chapter 2 Definitions

7 3. Methods and Examples - Chapter 2 Definitions Wave in Stockdon Equation is offshore wave Slope in Stockdon Equation is swash zone Figure 2.5 Wave setup terms (static (also called steady and average) and dynamic (also called infragravity) and incident wave runup. Adapted from MacArthur, et al,

8 3. Methods and Examples - Chapter 2 Definitions Figure 2.3 Typical cross section for TAW runup equation (FEMA, 2015) Figure 2.4 Plot of the TAW Runup Equations in Non-dimensional Form (FEMA, 2005)

9 Figure a. (top) shows how the components that influence TWL change across the surfzone, and 2.6b shows the Composite Slope Method that is applicable to most California coasts where wave setup from larger waves maximize Total Water Levels (FEMA, 2005) 3. Methods and Examples - Chapter 2 Definitions

10 3. Methods and Examples- Chapter 2 Definitions Figure 2.7 Cumulative distribution of total water level at Ocean Beach, San Francisco comparing SIO GCM (green) and real data (red) calculated with the Stockdon equation and a modified TAW approach with real data (blue).

11 3. Methods and Examples - Chapter 3 Methods TMM provides guidance to modify FEMA flood maps for future sea levels. There are four levels of application that entail a range of effort and information. The lower levels of application are simpler to apply and the adjustments to the future conditions hazards information are limited. Higher levels require more effort but greater accuaracy. Higher levels require more information and capability. The levels are: 1. Comparison between FEMA flood limits and future projections. 2. Adjust FEMA V-Zones to include effects of sea level rise. 3. Address other hazard zones and geomorphic processes 4. Apply FEMA methods to SIO or other future conditions outputs.

12 3. Methods and Examples - Chapter 3 Methods Level 2: Adjust V-Zones: Adjustment of the V-Zone for future conditions will typically be the most useful application owing to the high hazards and insurance premiums, and restrictive building requirements associated with this zone. Depending on the data available, there are two recommended Level 2 methodologies, identified below and described in Section 3.2: Level 2.a: Add sea level rise: Add sea level rise to TWL and apply geomorphic adjustment; and, Level 2.b: Prorate Components: Prorate existing water level by adding future change and prorate wave runup by multiplying by ratio of future change, and sum to get future TWL. Apply geomorphic adjustment. Geomorphic adjustment is the change in shore geometry resulting from sea level rise, primarily due to waves breaking on the shore at a higher elevation, and associated erosion and sediment transport.

13 3. TMM Organization - Chapter 3 Methods 2a Eq (1) With a Morphology Function TWLfuture (time) = TWLexisting + SLR(time) * F(Morphology, time) F(Morphology, time) = 1 for erodible backshores (can use Stockdon runup eq with landward migrated shore Figure 3.2) F(Morphology, time) = 1 to 4, with a default of 2, for static (erosion resistant) backshores (can use modified TAW methodology with landward overtopping extent Figure 3.4). This formulation is similar to that developed for the FEMA Pilot Study (BakerAECOM, 2016; Vandever et al., 2016), which uses the term Amplification Factor instead of Morphology Function. TABLE 3.1 MORPHOLOGY FUNCTION SUMMARY Backshore Waves Morphology Function (MF) values, ΔTWL=(MF)*SLR Explanation and simplifying assumption Erodible 1.0 Shore adjusts to sea level rise, runup does not change Erosion resistant non-breaking waves 1.0 Runup does not change Erosion resistant breaking waves default values 2.0 to Erosion resistant breaking seas 2.0 to Erosion resistant breaking swell 3.0 to Backshore cannot adjust, runup is amplified: Intermediate range and value Backshore cannot adjust, runup is amplified: High steepness seas have lower relative runup Backshore cannot adjust, runup is amplified: Low steepness swells have higher relative runup

14 3. TMM Organization - Chapter 3 Methods 2b Eq. (6) TWL future = SWL FEMA + {RWL future -RWL existing }+ R FEMA * {R future /R existing } Where {RWL future -RWL existing } = increase in reference water level predicted based on climate projections And {R future /R existing } = relative increase in wave runup predicted based on climate projections Eq. (7) R FEMA = TWL FEMA - SWL FEMA RECOMMENDATIONS: existing and future studies should provide these parameters. In the meantime: Select SWL FEMA from FEMA FIS backup, from SIO data (Appendix B1), from other source(s) (Appendix B2) Solve for R FEMA or attain from FEMA FIS backup Attain{R future /R existing } from SIO (Appendix B1) or other Future Conditions forecast Attain {RWL future -RWL existing } from SIO (Appendix B1) or other Future Conditions Forecast (can be simplified to = Sea-level Rise) Consider Morphology Response (shore recession and runup amplification) if not included in {R future /R existing }

15 3. Methods and Examples - Chapter 3 Methods Figure 3.5 Bore Propagation Driven by Wave Runup above the Shore Elevation Figure 3.6 Expanded inland extent of wave action due to increased overtopping for a range of negative freeboard of ΔRfuture/ΔRexisting between 1.1 and 3 and Yexisting between 5 and 100 feet.

16 3. Methods and Examples- Chapter 3 Methods Figure 3.7 Landward extent of wave runup for Bay (dashed; T=5s) and Open Coast (solid; T=15s) conditions using the Cox and Machemehl and Composite Slope models

17 3. Methods and Examples - Chapter 3 Methods Eq (2) Shore Recession = SR = a * {s / m) where s = sea level rise m = shore face slope a = dune reduction factor = shore face depth / (effective dune height + shore face depth) Figure 3.2 plots typical values of SR/s using a shore face depth of 40 feet. Effective dune height is the height of the backshore above the beach, multiplied times the percent of the material that is beach sand: The minimum practical value of a recommended in this analysis is 0.67 based on a dune height equal to the shore face depth and a beach sand content of 50% by volume. In the legend, the first number is the inverse of the slope (e.g. 20 indicates a slope of 1/20=0.05) and the second number is a. Note that a does not have a great effect for the steeper slopes Figure 3.2 Plot of Relative Shore Recession for a Shore Face Depth of 40 Feet and a Range of Reduction Factor a Values Associated with Backshore Sand Contributions

18 3. Methods and Examples - Chapter 4 Examples Figure 4.3 Level 2a Example, 3 of SLR Location 1: Zone VE elevation increased to 25 NAVD; Zone VE extent moved 180 landward. Location 2: Zone VE elevation increased to 32 NAVD; Zone VE extent moved 70 landward Method 2a results 25 (+3 = SLR) and 32 (+6 = 2 x SLR)

19 3. Methods and Examples - Chapter 4 Examples Profile B: Rivera (Dune) Existing Future Profile A: South of Sloat (Armored) Existing Future Figure 4.4 Profiles of the shore and near-shore of Ocean Beach in the area at Rivera Street, characterized by a sandy dune backshore (Location One). Data shown is a composite of Lidar data collected by NOAA and ground and bathymetric survey data collected by the USGS. Figure 4.5 Profiles of the shore and near-shore of Ocean Beach in the area south of Sloat Boulevard, characterized by an armored bluff backshore (Location Two). Data shown is a composite of Lidar data collected by NOAA and ground and bathymetric survey data collected by the USGS. Method 2b results using prior calculations by ESA (2012) for similar locations, and using SLR = 4.6 (interim California guidance for high sea level rise scenario) 27 (vs 22 ) and 44 (vs 26 ) MR / AMP Factor of ~ 1 and ~ 4, respectively Conclusion: Method 2a default may underestimate TWL for erosion-resistant morphology (MR / AMP of 2 to 3 versus computed 4)

20 4. TMM Recommendations 1. FEMA existing conditions studies should include the still water levels (SWL) and wave runup values (R) for each total water level (TWL) used to define high velocity zones (V ZONES). This may require additional work, to calculate and select the SWL and R values most appropriate for the 100-year TWL. Other information, such as the shore profile geometry, equations used, and profile parameters such as slopes should also be provided. 2. Future conditions flood studies should provide the TWL, SWL and R values for mapped or otherwise designated location. Other information, such as the shore profile geometry, equations used, and profile parameters such as slopes should also be provided. 3. Both FEMA existing conditions studies and future conditions studies by FEMA or others should provide guidance on how the study results can be related to future conditions, and existing FEMA maps, respectively. 4. Studies should characterize the existing backshore conditions in terms of the morphology function defined in this Technical Methods Manual. The required information consists of whether the shore is erodible or not, and the type of wave condition driving the hazard (breaking or not, long period swell or short period seas). Table 3.1 (repeated below as Table 5.1) provides an overview of the Morphology Function concept and suggested values. Note that in most cases, the total water level (TWL) increases (ΔTWL) more than the amount of sea level rise (SLR), unless the shore adjusts landward, with the exception being for non-breaking waves. Adaptation measures are not included in this analysis. 5. Coastal erosion should be considered in addition to flooding, along with estimates of the increase in erosion rates with sea level rise. 6. Additional attention needs to be applied to develop specific guidance for quantifying future coastal hazards, building upon the substantial progress made to date by California, NOAA and others. The gap to be filled or bridged is between the substantial progress made by science informing policy and educating the public, and the needs at the planning and engineering applications level. While requiring a multi-discipline effort, greater participation by engineers is required to develop practical solutions needed to facilitate informed planning and resilient design.

21 4. TMM Recommendations TABLE 5.1 MORPHOLOGY FUNCTION RECOMMENDATIONS Backshore Waves Morphology Function (MF) values, ΔTWL=(MF)*SLR Explanation and simplifying assumption Erodible 1.0 Shore adjusts to sea level rise by migrating landward, runup does not change Erosion resistant non-breaking waves 1.0 Runup does not change Erosion resistant breaking waves default values 2.0 to Erosion resistant breaking seas 2.0 to Erosion resistant breaking swell 3.0 to Backshore cannot adjust, runup is amplified: Intermediate range and value Backshore cannot adjust, runup is amplified: High steepness seas have lower relative runup Backshore cannot adjust, runup is amplified: Low steepness swells have higher relative runup

22 Speaker: Bob Battalio End slide Professional Civil Engineer (CA,WA,LA,OR, FL) Coastal Processes training from UC Berkeley, 1985 Chief Engineer, Vice ESA, San Francisco Engineering Criteria Review Board, BCDC Practices Coastal Zone Engineering and Management Vice President, California Shore and Beach Preservation Association (Non profit) Surfer Photographs copyright Colin Brown